For the direct reduction (DR) of iron oxide to sponge iron, the reducing agents H2 and CO, or any combination of them, can be used. Normally, a hydrocarbon source, such as natural gas, is utilized to produce these reducing agents via a catalytic or non-catalytic reforming process. The most practiced reforming processes are catalytic steam/CO2 reforming in a tubular reformer with an external heat supply, autothermal reforming (ATR) in a packed catalyst bed with internal heat supplied by the partial combustion of the hydrocarbon(s) with oxygen, and a non-catalytic partial oxidation (PDX) reforming process whereby the hydrocarbon source is partially oxidized and reformed to a mixture of H2 and CO, while internally providing the required heat for the reactions.
Besides natural gas, another widespread source of hydrocarbons in the iron and steel industry is COG, which typically contains 20-28% methane. Due to its considerable CH4 content, COG can be reformed into H2 and CO to produce reducing gas to reduce iron oxide to metallic iron, in the form of direct reduced iron (DRI), hot direct reduced iron (HDRI), or hot briquetted iron (HBI) in a DR plant. A typical COG stream coming from a COG treatment plant contains between 50.0-65.0% H2, 4.0-8.0% CO, up to 2.0% benzene-toluene-xylene (BTX), and up to 5.0% higher hydrocarbons, such as ethane, propane, and ethylene. The data available for different plants in China, Japan, and Germany indicate that the minimum concentration of H2 in a typical COG stream is 52.0%. The presence of such high concentrations of H2 in COG has adverse consequences for both catalytic and non-catalytic reforming processes, since it is the main product of reforming reactions; and, therefore, reduces the efficiency of the reforming reactions. In other words, since the rate of reforming reactions is slower in the presence of high concentrations of hydrogen, more energy is consumed to reform the hydrocarbons to H2 and CO.
More specifically, for ATR and PDX processes, the presence of high amounts of H2 has another undesired outcome, i.e. this useful reducing agent is combusted to form H2O, an oxidizing agent. In other words, while the hydrocarbon source is being reformed by oxygen to produce H2 (and CO), the already present H2 consumes loads of available oxygen to produce H2O, which translates into wasting material and energy resources.
Thus, the present invention provides an economic DR process in which the majority of the H2 and CO is recovered from the COG before sending it to the PDX unit. Therefore, the overall efficiency of the plant improves considerably in terms of material and energy consumption, and capital and operating investments (i.e. CAPEX and OPEX).